A high electron mobility transistor (HEMT) has an excellent high-speed performance, and therefore, HEMT is applied for a signal processing circuit for an optical communication system, other high-speed digital circuits, for example. In particular, HEMT has an excellent low-noise performance, and therefore, an application for an amplifier at microwave band or millimeter wave band has been expected. To obtain a sufficient amplifier gain when the amplifier is operated at the millimeter wave band, it is effective to increase a current gain cut-off frequency (fT). It is possible to increase the current gain cut-off frequency (fT) by reducing a capacitance between a gate and a source.
A low dielectric constant film is used for an interlayer insulating film to reduce a parasitic capacitance between wirings. For example, low dielectric constant materials such as benzocyclobutone (BCB) and polysilazane are used for an interlayer insulating film of a monolithic microwave integrated circuit (MMIC).
It is also discussed to form a cavity around a gate electrode in an interlayer insulating film to improve high-frequency characteristics of a HEMI.
However, when a cavity is formed, strength of an interlayer insulating film is lowered, and it is easy to be affected by transmission of moisture from outside.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2004-95637
Patent Literature 2: Japanese Laid-open Patent Publication No. 2006-210499
Patent Literature 3: Japanese Laid-open Patent Publication No. 05-335343
Patent Literature 4: Japanese Laid-open Patent Publication No. 2009-272433